This invention relates to methods for drying sludges and more particularly provides methods for continuous drying of sludges in rotary screw type indirect heat exchangers.
Drying of sludges is a common process in numerous applications. Examples ranges from the treatment of wastes such as paint sludge, to the drying of blood cells, to the recovery of ores, to the processing of foodstuff, among many other applications. The degree of drying also can encompass a wide range, for example, from the volumetric reduction of a sludge for use in subsequent process steps or disposal to a more complete drying resulting in a dry particulate product.
A common occurrence in the drying process, particularly where a substantial degree of drying is desired, is the caking of particulate matter on the surfaces of the heat exchanger. Caking oftentimes occurs in the drying of sludges in rotary screw type material conveying heat exchangers. The caking is often so complete as to make the conveyor appear as a cylinder or log, completely stopping the conveying action. Thus, caking requires that the process be shut down and the heat exchanger cleaned prior to continuation of drying. This batch type operation is costly and time consuming. Further, the methods and tools used to clean the heat exchanger can cause damage or excessive wear.
Many different structures and processes have been used for cleaning of the caked material from the heat exchanger surfaces. In some cases the surfaces, presenting a screw type profile on a central shaft, have been scraped manually with special tools or abrasive materials. This is very time consuming. In other cases the process is stopped and a scouring particulate material, such as rock salt, has been placed into the caked unit and run through the unit to abrasively remove the caked material from the heat transfer surfaces. These processes, while an improvement over manual scrapping, still require periodic shutdown of the sludge drying process and continuation only on a batch by batch basis.
In some systems, complex mechanical devices have been used to perform a mechanical wiping of the heat transfer surfaces simultaneously with the drying process. Such systems are complex and prone to failure, and still tend to require periodic shutdown for ultimate cleaning. An example of a mechanical cleaning structure is given in U.S. Pat. No. 3,808,701. There, a drying unit includes a central rotor having a helical band and also scraping and wiping elements which extend to within a close clearance of the inner containing wall. The wiping and scraping elements engage agglomerates which form on the wall to remove them. Although this configuration helps to provide a more uniform product, there remains a likelihood of caking of the material on the helical band.
Another mechanical configuration includes dual "self-cleaning" screws so closely oriented so as to scrape buildup from the heat exchange surfaces of the adjacent screw. The critical nature of the spacing makes such units costly to fabricate.
A process for cleaning conduits, including heat exchanger tubes, is described in U.S. Pat. No. 4,579,596. A nonagglomerating drying agent is concurrently mixed with cleaning particles entrained in a carrying fluid. The mixture, in a stated improvement of the Sandjet process, is introduced into a conduit at a high velocity to achieve desired cleaning. A similar mixture could be used to clean a helical screw heat exchanger having caked product on its surfaces. A primary limitation of such system is, however, the requirement that the operation be interrupted to perform the cleaning.
A somewhat similar cleaning method proposed for cleaning extruders is described in U.S. Pat. No. 3,776,774. In that teaching, two polymers are inserted into the barrel of an extruder. One is particularly brittle and is crushed in the extruder barrel, tending to clean the inside of the barrel. The second polymer melts at a lower temperature than the crushed material and, after melting, helps to remove the crushed polymer and loosened deposits from the extruder barrel. While similar materials could also be used with a screw type indirect heat exchanger, they still require periodic interruption of the drying process in order to perform the cleaning.
U.S. Pat. No. 4,193,206 describes a process for drying sewage sludge. One embodiment of that teaching uses a rotating helical screw conveyor element surrounded by a porous wall which functions as a mechanical dewatering zone for the sludge. A plasticizer material is added to the sludge being processed. Also added to the sludge is a stream of recycled dry solids. The admixture of the plasticizer and the dry material with the incoming wet sludge helps to provide a product stream with a desired bulk density that is more readily processed in an extruder. The recycled product is comprised of the fine solids contained in the sludge material. Undesirable product buildup can also occur on units operated in this manner.
It is therefore desirable to provide a method for operating screw type indirect heat exchangers which alleviates limitations caused by caking. It is particularly desirable to provide methods which eliminate the need for complex mechanical structures. It is also desirable to provide methods which allow for increased operating time. Particularly useful are methods which avoid caking and/or which allow continuous removal of any caked materials. It is further desirable to provide operating and/or cleaning processes which do not add undesirable materials to the dried product material where an uncontaminated product is required. It is also desirable to provide sludge drying methods which add flexibility to the control of the rate of drying and other related process parameters.